The most widely diffused solid polymeric electrolytes are those based on a polyethylene oxide, or on another polyether, such as, for example, those as disclosed U.S. Pat. No. 4,471,037; FR 2,523,769; FR 2,568,574; and EP 13,199.
Such materials display interesting values of ionic conductivity only at relatively high temperatures, at which they unfortunately display a low mechanical strength: they are consequently not really interesting for such practical uses as electrolytic separators in electrochemical devices, optical and electrochromic display and in sensors.
In U.S. Pat. No. 5,173,205 and IT 1,222,929 particular polymeric polyvinyl ether-based electrolytes are disclosed which display both an improved mechanical strength and a satisfactory conductivity even at relatively low temperatures.
Preparing such materials requires complex multistep processes comprising the copolymerization of suitable vinyl ethers at a temperature of the order of -75.degree./-80.degree. C. for a time comprised within the range of from 30 to 60 minutes, dissolving the resulting solid, crosslinked polyvinyl ether in a suitable solvent, blending said solution with a solution containing an ionic compound and evaporating the solvent in order to obtain a membrane.
In U.S. Pat. No. 5,173,205, the suitably functionalized polyvinyl ether is crosslinked by means of a diprotic crosslinker agent during the step of polymer blending with the ionic compound. This step is followed by solvent evaporation in order to obtain the membrane.